Specifying glass partitions? Your guide to understanding sound

To understand how glass walls perform acoustically, it helps to start with the decibel scale. Decibels measure sound level, while STC is used to rate how effectively a wall assembly reduces airborne sound transmission.

The decibel scale is logarithmic rather than linear. A 10 dB increase represents a tenfold increase in sound intensity, while a 20 dB increase represents a hundredfold increase.

Typical sound levels can include:

• A whisper: around 30 dB
• Normal conversation: around 60 dB
• A vacuum cleaner: around 70–80 dB
• A lawn mower: around 90 dB
• A car horn: around 100–110 dB
• A jet engine at close range: around 120–130 dB

Actual sound levels vary depending on the source, distance and surrounding environment.

Defining Acoustic Performance

Glass wall assemblies are acoustically rated through controlled laboratory testing. In the US, airborne sound transmission loss through building partitions is commonly measured in accordance with ASTM E90, with the resulting data classified using ASTM E413 to determine the Sound Transmission Class (STC) rating.

During testing, the wall assembly is installed between two acoustically isolated rooms. Sound is generated on one side and measured on the other across a range of frequencies. The resulting transmission-loss data is then used to determine the STC rating.

A higher STC rating generally indicates greater resistance to airborne sound transmission. The rating applies to the specific assembly and configuration tested, including the glass, framing, seals and other components included in that test.

Understanding Sound On-Site

Laboratory testing is carried out under controlled conditions. Completed buildings are more complex, with sound able to travel around, above or below a wall through what are known as flanking paths.

Typical sources of sound leakage include:

• Electrical and service penetrations through walls, floors and ceilings
• Sound transmission above suspended ceilings
• Ductwork crossing the wall line
• Gaps where the glass wall meets adjacent walls, floors or façades
• Sound traveling through adjoining building elements or the building façade

Installation quality and the interfaces between the glass wall and adjoining construction can therefore have a significant influence on the acoustic performance experienced in the finished space.

Where sound isolation between completed rooms is measured on site, ASTM E336 provides the relevant field test method.

Understanding STC Ratings

STC provides a useful way to compare the laboratory-tested sound insulation of different wall assemblies.

As a broad guide:

STC 25 – Normal speech can generally be heard and understood.
STC 30 – Loud speech can generally be heard clearly.
STC 35 – Loud speech may still be distinguishable.
STC 40 – Loud speech may be audible but becomes more difficult to understand.
STC 45 – Loud speech is significantly reduced.
STC 50 – Loud speech is generally difficult to hear through the wall.

These descriptions are general guidance rather than guaranteed levels of speech privacy. Actual performance will depend on factors such as background noise, room finishes, doors, ceilings, junctions and other sound paths within the building.

For project specifications, the required STC should therefore be established as part of the wider acoustic design rather than selected from the glass wall rating alone.